Rolling bearing, material for rolling bearing, and equipment having rotating part using the rolling bearing

ABSTRACT

The invention relates to a rolling beating which provides superior noiselessness and high corrosion resistance and longer life, and is manufactured at a low cost, a material for the rolling bearing, and an instrument including a rotating portion using the rolling bearing. A plurality of rolling elements  3  are provided between an inner ring  2  and an outer ring  1 . At least one of the inner and outer rings  2  and  1  is formed of corrosion resistant bearing steel comprising a specific chemical component. An average value of a circle equivalent diameter of eutectic carbides contained in the corrosion resistant bearing steel is 0.2 to 1.6 μm. An average area of the eutectic carbides is 0.03 to 2 μm 2 . An area ratio of the eutectic carbides is 2 to 7%. The hardness of the corrosion resistant bearing steel is HRC 58 to 62 by JIS. The amount of retained austenite in the corrosion resistant bearing steel is 6 volume % or less.

TECHNICAL FIELD

The present invention relates to a rolling bearing, a material for therolling bearing, and an instrument having a rotating portion using therolling bearing. More particularly, the present invention relates to arolling bearing suitable for use in a rotating portion of a precisioninstrument such as videotape recorder, or a computer peripheral device,a material for the rolling bearing, and the instrument having therotating portion using the rolling bearing.

BACKGROUND ART

Conventionally used bearing steels are described below.

Rolling bearings such as ball bearings and roller bearings, which have acontact surface strength in a range of 1000 to 1300 MPa or in a range of3000 to 4000 Mpa, are formed of high-carbon chromium bearing steelhaving a high content of carbon or case-hardened steel having acarburized surface. The high-carbon chromium bearing steel contains maincomponents of 1.1% by weight of carbon and 1 to 1.5% by weight ofchromium and varies its quenching characteristic according to thecontent of manganese and molybdenum. This type of steel is quenched at atemperature of 1050 to 1120K, and then tempered at a temperature of 420to 470K to produce a texture with 7 to 8% by weight of sphericalcementites dispersed in martensite. Since the hardness of the temperedsteel is as high as HRC 58 to 64 by Japanese Industrial Standard(hereinafter referred to as JIS), clean steel having less flaws and lessnon-metal inclusions is desirable. At present, the high-carbon chromiumbearing steel is typically manufactured utilizing deoxidization bycarbon under vacuum degassing process. Further, special meltingprocesses such as electroslag remelting or vacuum arc remelting arecombined to reduce non-metal inclusions and to provide fine texture.

The carburized bearing is manufactured by carburizing case-hardenedsteel, and therefore, has high surface hardness and flexible coreportion. The carburized bearing is especially suitable for use as abearing which is subjected to an impact load.

In case of bearings used under a temperature which is above 390K, thetexture of steel tempered at a low temperature is subject to variation,thereby resulting in softening or variation in dimension, and thus, thesteel becomes unusable. For this reason, high-carbon high-alloy steelswhich are tempered at high temperatures, such as M50 (0.8 wt % C—4 wt %Cr—4.3 wt % Mo—1 wt % V) or T1 (0.7 wt % C—4 wt % Cr—18 wt % W—1 wt % V)are used.

However, the conventional bearing steels suffer from drawbacks asdescribed below.

The case-hardened steel is difficult to reduce oxygen content in termsof melting and refining in contrast to the high-carbon chromium bearingsteel and tends to generate oxide-based non-metal inclusions, which mayreduce rolling contact life.

In addition, the high-carbon high-alloy steel tends to generatelarge-sized carbides, which may also reduce rolling contact life of thebearing.

On the other hand, the high-carbon chromium bearing steel does not havesuch drawbacks and can obtain high processing precision. Therefore, thehigh-carbon chromium bearing steel is suitable for use as the rotatingportion of the precision instrument which particularly requiresnoiselessness during rotation. However, the high-carbon chromium bearingsteel tends to rust and needs to be coated with rust-proof oil on anouter surface thereof. The rust-proof oil may be gasified and cause amalfunction of the precision instrument.

Accordingly, martensite based stainless steel corresponding to SUS440Csteel by JIS with high corrosion resistance and high wear resistance isused for the bearings used in corrosive atmosphere. However, thisstainless steel contains eutectic carbides resulting from an eutecticreaction when molten steel is solidified or non-metal inclusions such asalumina resulting from a chemical reaction of impurities of a materialin the molten steel. When this stainless steel product is cut,high-precision cutting action cannot be achieved due to a difference incutting state (machinability) of texture between the eutectic carbidesor the non-metal inclusions and the stainless steel product. Inparticular, since rolling contact grooves formed on inner and outerrings cannot be processed with high precision, the rolling bearingvibrates and generates a high level of noise during rotation. Therefore,this stainless steel product cannot be used for the rotating portion ofthe precision instrument.

Accordingly, there has been proposed a rolling bearing which can improvenoiselessness, and provides high wear resistance and high corrosionresistance (for example, see Japanese Unexamined Laid-Open PatentPublication No. Hei 6-117439 and Japanese Patent Publication No. Hei5-2734.

The Publication No. Hei 6-117439 discloses a ball bearing comprising aplurality of balls made of high-carbon chromium bearing steel interposedbetween inner and outer rings, at least one of the inner and outer ringsbeing formed of martensite based stainless steel having hardness of HRC58 or higher by JIS and comprising eutectic carbides having a diameterof 10 μm or less.

The Publication No. Hei 5-2734 discloses a rolling bearing formed ofstainless steel comprising a plurality of rolling elements interposedbetween inner and outer rings, the stainless steel comprising carbon of0.6 to 0.75 wt %, silicon of 0.1 to 0.8 wt %, manganese of 0.3 to 0.8 wt%, chromium of 10.5 to 13.5 wt %, iron as remaining component, andimpurities inevitably incorporated thereinto, and containing eutecticcarbide with a long diameter of 20 μm or less and an area ratio of 10%or less.

When large-sized eutectic carbides appear on the surface of the bearing,a proper finished surface is difficult to form, due to the difference incutting state between these carbides and a matrix around the carbides,and a noise may be generated during rotation, as described above. Inaddition, since the large-sized eutectic carbides generate a differencein wear resistance between the carbides and the matrix around thecarbides during use of the bearing, they drop from a cracked surface,which causes a deformation of the surface shape of the bearing andsignificantly degrades noiselessness. It is therefore desirable tominimize the size of the carbides, because they are less likely toappear on the surface of the bearing. So, reducing the diameter of thecarbide to 20 μm or less or 10 μm or less as disclosed in the abovepublications is effective in lowering the level of noise. Nonetheless, asatisfactory noiselessness cannot be obtained merely by reducing thesize of the carbides. In addition, additional processes in manufacturingtechnique are required to thus reduce the size of the carbides. Thisgreatly increases a manufacturing cost and is therefore unpractical.

The present invention has been developed in view of the above describedproblems accompanied by the prior arts, and an object thereof is toprovide a rolling bearing which can improve noiselessness, and providehigh corrosion resistance and longer life (corresponding to high wearresistance) and can be manufactured at a low cost, a material for therolling bearing, and an instrument including a rotating portion usingthe rolling bearing.

DISCLOSURE OF THE INVENTION

The present invention has been made as follows: attention has beenfocused on an average value of circle equivalent diameters of chemicalcomponents in a steel, specifically, eutectic carbides contained incorrosion resistant bearing steel, an average area of the eutecticcarbides, an area ratio of the eutectic carbides, hardness of thecorrosion resistant bearing steel, the amount of retained austenite inthe corrosion resistant bearing steel, and an average crystal grain sizeof the corrosion resistant bearing steel, and the relationship betweenthese numeric values and cutting state (machinability, processability),noiselessness of an instrument including the rolling bearing or arotating portion, life of the rolling bearing, a manufacturing cost ofthe rolling bearing, etc, have been intensively studied.

Specifically, by relatively reducing the contents of carbon and chromiumfor improving the life or corrosion resistance, it is possible toinhibit generation of the eutectic carbides. And, in order to resolvethe problem caused by reducing the content of carbon and chromium,copper and molybdenum are added in relatively large amount.

It is clear that the noiselessness of the rolling bearing is effectivelyimproved by reducing a maximum diameter of the eutectic carbides.Actually, however, it is difficult to reduce the maximum diameter of theeutectic carbides with a general manufacturing technique for massproduction, and additional processes are necessary, which significantlyincreases a manufacturing cost. Considering the manufacturing cost, itwould be preferable that the average value of the circle equivalentdiameters of the eutectic carbides, the average area of the eutecticcarbides, and the area ratio of the eutectic carbides are set withinpredetermined ranges, because machinability is substantially improvedand the manufacturing cost is not increased.

Typically, the average value of the circle equivalent diameters of theeutectic carbides is approximately 2.0 to 2.8 μm. Reducing the averagevalue of the circle equivalent diameters is effective in improving themachinability, and it would be preferable that the average value of thecircle equivalent diameters of the eutectic carbides is 0.2 to 1.6 μm.In this application, the average value of the circle equivalentdiameters of the eutectic carbides means an average value of diametersof circles into which areas of the respective eutectic carbides obtainedby an image analysis device have been converted.

The average area of the eutectic carbides is typically approximately 3.0to 6.0 μm². Reducing the average area is effective in improvingmachinability, and hence it would be preferable that the average area ofthe eutectic carbides is 0.03 to 2.0 μm².

It would be preferable that an absolute content of the eutectic carbidesis limited to improve the machinabiliy. Therefore, it would bepreferable that the area ratio of the eutectic carbides is 2 to 7%. Inthis application, the area ratio of the eutectic carbides means a ratio(percentage) of a total area of the eutectic carbides to a total areameasured within a field of view.

It would be preferable that the hardness of the corrosion resistantbearing steel is 58 to 62 in Rockwell hardness C scale (HRC) by JIS inorder to ensure longer rolling contact life, wear resistance andtoughness of a raceway surface or a rolling contact surface.

It would be preferable that the content of retained austenite is 6volume % or less, because it is necessary to reduce the content ofretained austenite to inhibit unwanted permanent deformation of theraceway surface or the rolling contact surface which may be caused by aload or an impact. By reducing the content of retained austenite,indentation resistance of the raceway surface or the rolling contactsurface is improved and time-lapse degradation of a surface smoothnessof the raceway surface or the rolling contact surface can be prevented.

Further, it would be preferable that an average crystal grain size iswithin a range of 6 to 9. 5 μm in order to stabilize processability andhardness.

A first invention of the present application provides a rolling bearingcomprising a plurality of rolling elements provided between inner andouter rings, at least one of the inner and outer rings being formed ofcorrosion resistant bearing steel comprising carbon of 0.5 to 0.56 wt %,silicon of 1 wt % or less, manganese of 1 wt % % or less, phosphorus of0.03 wt % or less, sulfur of 0.01 wt % or less, chromium of 8.00 to 9.50wt %, molybdenum of 0.15 to 0.50 wt %, copper of 0.30 to 0.7 wt %,titanium of 15 ppm or less, vanadium of 0.15 wt % or less, oxygen of 15ppm or less, iron as remaining component and impurities inevitablyincorporated thereinto, the corrosion resistant bearing steel containingeutectic carbides having a circle equivalent diameter with an averagevalue of 0.2 to 1.6 μm, the eutectic carbides having an average area of0.03 to 2 μm² and an area ratio of 2 to 7%, the corrosion resistantbearing steel having a hardness of HRC 58 to 62 by JIS, and containing aretained austenite of 6 volume % or less.

A second invention of the present application provides a rolling bearingcomprising a plurality of rolling elements provided between inner andouter rings, the inner and outer rings, and the rolling elements beingformed of corrosion resistant bearing steel comprising carbon of 0.5 to0.56 wt %, silicon of 1 wt % or less, manganese of 1 wt % or less,phosphorus of 0.03 wt % or less, sulfur of 0.01 wt % % or less, chromiumof 8.00 to 9.50 wt %, molybdenum of 0.15 to 0.50 wt %, copper of 0.30 to0.7 wt %, titanium of 15 ppm or less, vanadium of 0.15 wt % or less,oxygen of 15 ppm or less, iron as remaining component and impuritiesinevitably incorporated thereinto, the corrosion resistant bearing steelcontaining eutectic carbides having a circle equivalent diameter with anaverage value of 0.2 to 1.6 μm, an average area of 0.03 to 2 μm² and anarea ratio of 2 to 7%, the corrosion resistant bearing steel having ahardness of HRC 58 to 62 by JIS, and containing a retained austenite of6 volume % or less.

A third invention of the present application provides a rolling bearingcomprising a plurality of rolling elements provided between a rollingcontact groove formed on an outer periphery of a shaft and a rollingcontact groove formed on an inner periphery of an outer ring, at leastone of the shaft and the outer ring being formed of corrosion resistantbearing steel comprising carbon of 0.5 to 0.56 wt %, silicon of 1 wt %or less, manganese of 1 wt % or less, phosphorus of 0.03 wt % or less,sulfur of 0.01 wt % or less, chromium of 8.00 to 9.50 wt %, molybdenumof 0.15 to 0.50 wt %, copper of 0.30 to 0.70 wt %, titanium of 15 ppm orless, vanadium of 0.15 wt % or less, oxygen of 15 ppm or less, iron asremaining component and impurities inevitably incorporated thereinto,the corrosion resistant bearing steel containing eutectic carbideshaving a circle equivalent diameter with an average value of 0.2 to 1.6μm, the eutectic carbides having an average area of 0.03 to 2 μm² and anarea ratio of 2 to 7%, the corrosion resistant bearing steel having ahardness of HRC 58 to 62 by JIS, and containing a retained austenite of6 volume % or less.

A fourth invention of the present application provides a rolling bearingin which, in the first, second or third invention, an average crystalgrain size of the corrosion resistant bearing steel is 6 to 9.5 μm.

A fifth invention of the present application provides a material for therolling bearing, which is corrosion resistant bearing steel comprisingcarbon of 0.5 to 0.56 wt %, silicon of 1 wt % or less, manganese of 1 wt% or less, phosphorus of 0.03 wt % or less, sulfur of 0.01 wt % or less,chromium of 8.00 to 9.50 wt %, molybdenum of 0.15 to 0.50 wt %, copperof 0.30 to 0.7 wt %, titanium of 15 ppm or less, vanadium of 0.15 wt %or less, oxygen of 15 ppm or less, iron as remaining component andimpurities inevitably incorporated thereinto, the corrosion resistantbearing steel containing eutectic carbides having a circle equivalentdiameter with an average value of 0.2 to 1.6 μm, the eutectic carbideshaving an average area of 0.03 to 2 μm² and an area ratio of 2 to 7%.

A sixth invention provides an instrument having a rotating portion usingthe rolling bearing of the first or fourth invention.

A seventh invention provides an instrument having a rotating portionusing the rolling bearing of the second or fourth invention.

An eighth invention provides an instrument having a rotating portionusing the rolling bearing of the third or fourth invention.

A ninth invention is characterized in that the instrument having therotating portion defined in any one of the sixth to eighth inventions isa hard disk drive.

A tenth invention is characterized in that the instrument having therotating portion defined in any one of the sixth to eighth inventions isa precision instrument.

In this application, “noiselessness” means that “level of a noiseattributable to metal material in noises generated during an operationof a precision instrument such as a hard disk drive into which a rollingbearing comprising a rolling element, an inner ring or an outer ringformed by processing a metal material is incorporated is low.” The noiseis caused by vibration generated during rotation of the rolling bearing,and the vibration depends largely on a shape precision of the rollingelements, the inner ring and the outer ring as described previously. Ina relatively small-sized rolling bearing used in the area of precisioninstruments such as the hard disk drives, the noiselessness which isnegligible in other uses becomes significant.

Accordingly, at least one of the inner and outer rings of the rollingbearing comprising the plurality of rolling elements provided betweenthe inner and outer rings may be formed of the corrosion resistantbearing steel of the present invention, or at least one of the shaft andthe outer ring of the rolling bearing comprising the plurality ofrolling elements provided between the shaft provided with the rollingcontact groove on the outer periphery and the outer ring may be formedof the corrosion resistant bearing steel of the present invention.Thereby, the rolling bearing of the present invention is less likely torust than that formed of the high-carbon chromium bearing steel. As aresult, corrosion resistance and life of the bearing increase.

The corrosion resistant bearing steel may comprise carbon of 0.5 to 0.56wt %, silicon of 1 wt % or less, manganese of 1 wt % or less, phosphorusof 0.03 wt % or less, sulfur of 0.01 wt % or less, chromium of 8.00 to9.50 wt %, molybdenum of 0.15 to 0.50 wt %, copper of 0.30 to 0.70 wt %,titanium of 15 ppm or less, vanadium of 0.15 wt % or less, oxygen of 15ppm or less, iron as remaining component and impurities inevitablyincorporated thereinto, and a portion occupied by the eutectic carbides,i.e., area of the eutectic carbides may be set within a predeterminedrange. Thereby, machinability can be improved without a substantialincrease in manufacturing cost.

The average value of the circle equivalent diameters of the eutecticcarbides may be set to 1.6 μm or less, the average area of the eutecticcarbides may be set to 2 μm² or less, and the area ratio of the eutecticcarbides may be set to 7% or less. Thereby, the machinability can befurther improved, and the noiselessness can be significantly improved.

However, special manufacturing processes become necessary tosignificantly reduce the average value of the circle equivalentdiameters of the eutectic carbides, and the average area and the arearatio of the eutectic carbides. This significantly increases amanufacturing cost. If the average value of the circle equivalentdiameters of the eutectic carbides is 0.2 μm or more, the average areaof the eutectic carbides is 0.03 μm² or more, and the area ratio of theeutectic carbides is 2% or more, then the corrosion resistant bearingsteel can be manufactured substantially according to a normalmanufacturing processes. Therefore, economical manufacturing system canbe achieved without substantial increase in the manufacturing cost.

Further, if the inner and outer rings and the rolling elements areformed of the corrosion resistant bearing steel of the presentinvention, then strain caused by a difference in thermal expansioncoefficient will not occur during use under high-temperature condition,because the inner and outer rings and the rolling elements are formed ofthe same material. So, even under high-temperature condition,noiselessness and longer life can be achieved.

The hardness of the corrosion resistant bearing steel may be HRC 58 to62 by JIS. Thereby, longer rolling contact life, wear resistance andtoughness of the raceway surface or the rolling contact surface can beachieved.

The content of retained austenite in the corrosion resistant bearingsteel may be 6 volume % or less. Thereby, indentation resistance can beimproved, and time-lapse degradation of the surface smoothness of theraceway surface or the rolling contact surface can be prevented.

The average crystal grain size may be 6 to 9.5 μm. Thereby, themachinability, the noiselessness, the life, etc., of the corrosionresistant bearing steel can be improved.

When at least one of the inner and outer rings and rolling elements ofthe rolling bearing is formed of the corrosion resistant bearing steelof the present invention, the above effects can be obtained. Forexample, only the rolling elements may be formed of the corrosionresistant bearing steel of the present invention as the embodiment ofthe material for the rolling bearing of claim 5, and the inner and outerrings may be formed of stainless steel having the conventionalcomposition shown in table 3.

The content of component (weight %) of the corrosion resistant bearingsteel of the present invention is limited for the reason below.

Carbon is an essential element to provide high temperature strength andwear resistance. While the stainless steel disclosed in the publicationNo. Hei 5-2734 contains carbon of 0.6 to 0.75 wt %, the content ofcarbon is 0.5 to 0.56 wt % to inhibit generation of carbides in thepresent invention. The content of carbon which is 0.5 wt % or more isnecessary to ensure predetermined high temperature strength and wearresistance, but it would be preferable that the content of carbon is0.56 wt % or lower because large eutectic carbides are generated,machinability degrades, and corrosion resistance reduces if the contentis too high.

The contents of the following elements are set to predetermined valuesor less as follows: Silicon is 1 wt % or less, Manganese is 1 wt % orless, Phosphorus is 0.03 wt % or less, Sulfur is 0.01 wt % or less,Vanadium is 0.15 wt % or less, Titanium is 15 ppm or less, and Oxygen is15 ppm or less. These elements are set to these predetermined values orless to inhibit generation of non-metal inclusions without degradingmachinability, because, if the contents of these elements are too high,work hardening increases and hence the machinability degrades. Inaddition, if the contents of these elements are too high, quenchingcharacteristic disadvantageously degrades and the ratio of martensitedecreases.

The content of chromium, copper and molybdenum are limited to thepredetermined values for the reason below.

Chromium is bonded to C to form a carbide. Chromium increases wearresistance and corrosion resistance when homogeneously mixed in thematrix (formation of solid solution). In the present invention, sincecarbon is slightly reduced in contrast to the prior arts to inhibitgeneration of the carbides, chromium is correspondingly reduced to 8.00to 9.50 wt % as compared to the content (10.5 to 13.5 wt %) disclosed inthe publication No. Hei 5-2734. In order to resolve the problem which iscaused by reduction of the contents of carbon and chromium, copper andmolybdenum are added in relatively larger amount.

In brief, copper increases corrosion resistance and wear resistance.But, if copper is added too much, the steel is less likely to becracked. Therefore, it would be preferable that the content of copper is0.30 to 0.70 wt %.

Molybdenum increases quenching characteristic, prevents crystal grainfrom becoming coarse, and improves corrosion resistance. If molybdenumis less than 0.15 wt %, these effects cannot be substantially obtained,while molybdenum is more than 0.50 wt %, the steel containing muchmolybdenum cannot be quenched under known quenching conditions.Furthermore, since molybdenum is very expensive metal, cost increase ifMo is added in large amount.

The present invention is constituted as described above and provides arolling bearing with improved noiselessness, high corrosion resistance,and longer life, which can be manufactured at a low cost, a material forthe rolling bearing, and an instrument including a rotating portionusing the rolling bearing. Especially, the present invention contributesto cost reduction and improvement of noiselessness of a bearing forvideotape recorder, a computer peripheral device, etc., in particular,swing arm, thus offering remarkable industrial effects.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of an embodiment of a rollingbearing of the present invention;

FIG. 2 is a longitudinal sectional view of another embodiment of therolling bearing of the present invention;

FIG. 3 is a view showing the relationship between an average value (μm)of circle equivalent diameters of eutectic carbides and anderon value(M); and

FIG. 4 is a view showing the relationship between a maximum diameter(μm) of the eutectic carbides and cost index;

FIG. 5 is a view showing an scanning electron microscope (SEM) image ofa carbide deposited on corrosion resistant bearing steel or stainlesssteel; and

FIG. 6 is a perspective view of an external appearance of a hard diskdrive.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiments of the present invention will be described withreference to the drawings, but it should be appreciated that the presentinvention is not intended to be limited to the embodiments describedbelow.

Referring to FIG. 1, 1 denotes an outer ring, 2 denotes an inner ring,and 3 denotes a rolling element. A plurality of rolling elements 3 areloaded between a rolling contact groove 4 formed on an inner peripheryof the outer ring 1 and a rolling contact groove 5 formed on an outerperiphery of the inner ring 2.

A rolling bearing of example 1 of the present invention is as follows:

The outer ring 1 and the inner ring 2 are formed of corrosion resistantbearing steel of the present invention having a composition (weight %)shown in table 1 below, the rolling element 3 is formed of high-carbonchromium bearing steel (SUJ2 by JIS), and an average crystal grain sizeof the corrosion resistant bearing steel is limited within a range ofthe present invention.

A rolling bearing of example 2 of the present invention is as follows:

Only the outer ring 1 is formed of the corrosion resistant bearing steelhaving the composition shown in table 1, the inner ring 2 and therolling element 3 are formed of the high-carbon chromium bearing steel(SUJ2 by JIS), and the average crystal grain size of the corrosionresistant bearing steel is limited within the range of the presentinvention.

A rolling bearing of example 3 of the present invention is as follows:

The outer ring 1, the inner ring 2, and the rolling element 3 are formedof the corrosion resistant bearing steel having the composition shown intable 1.

As shown below, table 4 lists an area ratio of the eutectic carbidescontained in the corrosion resistant bearing steel, a maximum diameterof the eutectic carbides, an average value of circle equivalentdiameters of the eutectic carbides, an average area of the eutecticcarbides, Rockwell hardness C scale (HRC) by JIS of the corrosionresistance bearing steel, amount of retained austenite in the corrosionresistant bearing steel (volume %), and an average crystal grain size ofthe corrosion resistant bearing steel for each of the examples 1, 2, and3.

Referring to FIG. 2, a plurality of rolling elements 3 are loadedbetween a rolling contact groove 7 formed on an outer periphery of ashaft 6 and the rolling contact groove 4 formed on an inner periphery ofthe outer ring 1 (Example 4). In the example 4, the outer ring 1 and theshaft 6 are formed of the corrosion resistant bearing steel having thecomposition shown on table 1, and the rolling elements 3 are formed ofthe high-carbon chromium bearing steel (SUJ2 by JIS). Table 4 also listsan area ratio of eutectic carbides contained in corrosion resistantbearing steel, a maximum diameter of the eutectic carbides, an averagevalue of circle equivalent diameters of the eutectic carbides, anaverage area of the eutectic carbides, Rockwell hardness C scale (HRC)by JIS of the corrosion resistant bearing steel, amount of retainedaustenite in the corrosion resistant bearing steel (volume %), and anaverage crystal grain size of the corrosion resistant bearing steel forthe example 4.

While the outer ring 1 and the shaft 6 are both formed of the corrosionresistant bearing steel of the present invention in the example 4, onlyone of the outer ring 1 and the shaft 6 which requires corrosionresistance and high temperature strength may alternatively be formed ofthe corrosion resistant bearing steel depending on use conditions. In afurther alternative, the outer ring 1, the shaft 6, and the rollingelement 3 may be formed of the corrosion resistant bearing steel.

In example 5, the outer ring 1 and the inner ring 2 are formed ofcorrosion resistant bearing steel of the present invention having acomposition (weight %) shown in table 2 below, which is different fromthat shown in table 1, the rolling element 3 is formed of thehigh-corrosion chromium bearing steel (SUJ2 by JIS), and an averagecrystal grain size of the corrosion resistant bearing steel is limitedwithin the range of the present invention.

In example 6, all of the outer ring 1, the inner ring 2, and the rollingelements 3 are formed of the corrosion resistant bearing steel havingthe composition shown in table 2, and an average crystal grain size ofthe corrosion resistant bearing steel is limited within the range of thepresent invention.

Table 4 also lists an area ratio of eutectic carbides contained in thecorrosion resistant bearing steel, a maximum diameter of the eutecticcarbides, an average value of circle equivalent diameters of theeutectic carbides, an average area of the eutectic carbides, Rockwellhardness C scale (HRC) by JIS of the corrosion resistant bearing steel,amount of retained austenite in the corrosion resistant bearing steel(volume %), and an average crystal size of the corrosion resistantbearing steel for each of the examples 5 and 6.

To obtain the above corrosion resistant bearing steel, after conductingwater quenching from a temperature of 1025° C., sub-zero treatment at atemperature of −80° C. was conducted and then the steel was tempered to170° C. TABLE 1 (weight percent) C Si Mn P S Cr Cu Mo V Ti O 0.52 0.250.70 0.023 0.002 9.11 0.49 0.29 0.03 14 ppm 15 ppmFe = remaining componennt

TABLE 2 (weight percent) C Si Mn P S Cr Cu Mo V Ti O 0.50 0.25 0.500.010 0.003 8.85 0.51 0.40 0.04 13 ppm 12 ppmFe = remaining componennt

TABLE 3 (weight percent) C Si Mn P S Cr Mo V Ti O 0.68 0.85 0.64 0.0210.016 12.20 0.17 0.11 14 12 ppm ppmFe = remaining componennt

TABLE 4 Corrosion Resistant Bearing Steel Other Characteristics CarbidesAmount of Average Rolling Bearing Area Maximum Circle Equivalent AverageRetained Crystal Characteristic Values ratio Diameter Diameter AverageArea Hardness Austenite Grain Size Anderon Value Process- (%) (μm) (μm)(μm²) (HRC) (%) (μm) M H ability Life Cost Example 1 2.6 13 0.2 0.03 584 8.62 2.36 1.80 109 108 95 Example 2 3.1 12 0.6 0.3 60 6 7.46 2.40 1.92108 108 95 Example 3 2.4 17 1.5 1.8 59 4 10.2 2.60 1.98 108 105 95Example 4 4.6 15 1.1 1.0 62 5 11.4 2.63 1.99 106 106 95 Example 5 5.2 170.7 0.4 62 6 7.0 2.44 1.85 108 108 95 Example 6 2.8 12 1.0 0.8 60 4 6.72.45 1.90 105 106 95 Contrast 1 2.6 12 1.8 2.5 60 6 8.7 2.76 2.12 102103 132 Contrast 2 2.0 8 0.2 0.1 62 7 11.3 2.52 1.90 104 103 152Contrast 3 7.3 35 2.8 6.2 59 7 9.5 3.47 2.94 99 102 94 Prior Art 1 2.515 2.7 5.7 58 6 9.8 2.95 2.55 100 100 100 Prior Art 2 2.2 8 2.1 3.5 60 710.5 2.90 2.25 102 102 138

The area ratio of the eutectic carbides in the corrosion resistantsteel, the maximum diameter of the eutectic carbides, the average valueof the circle equivalent diameters of the eutectic carbides, and theaverage area of the eutectic carbides can be controlled according tomanufacturing conditions (e.g., refining time, degassing condition,incorporation of a diffusion heat treatment process, etc.) inmanufacturing processes including control of impurity elements,preparation of a material, refining, casting etc. However, in order toallow the conventional stainless steel having the composition (weight %)shown in table 3 to contain the eutectic carbides with a maximumdiameter of 20 μm or less, manufacturing cost may significantlyincrease, because of the use of a special material and an increase inthe manufacturing processes.

The average crystal grain size of the corrosion resistant bearing steel,the hardness of the corrosion resistant bearing steel, and the amount ofretained austenite in the corrosion resistant bearing steel can becontrolled based on heating temperature and heating time in quenching,cooling speed, cooling medium, cooling temperature and cooling time,tempering temperature and tempering time, etc.

An evaluation test of vibration and noise regarding the rolling bearingsof the examples 1 to 6 was conducted according to AFBMA (TheAnti-Friction Bearing Manufactures Association, Inc.) standard. Table 4also lists indices representing results (anderon values) of theevaluation test of vibration and noise, indices indication ofprocessability (machinability), life, and cost of the rolling bearingsof the examples 1 to 6.

Table 4 also lists results of evaluation of the contrasts 1 to 3 and theprior arts 1 and 2. The stainless steel used for these have theconventional composition which is shown in table 3 and is significantlydifferent from those of the corrosion resistant bearing steel of thepresent invention shown in tables 1 and 2. In the contrasts 1 to 3, therolling elements 3 are formed of high-carbon chromium bearing steel(SUJ2 by JIS), the outer ring 1 and the inner ring 2 are formed of thestainless steel having the composition shown in table 3, and at leastone of characteristic values of the area ratio of the eutectic carbidescontained in the stainless steel, the average value of the circleequivalent diameters of the eutectic carbides, the average area of theeutectic carbides, and the amount of retained austenite in the steel(volume %), is outside of the range of the present invention.

In the prior arts 1 and 2, the outer and inner rings 1 and 2, and therolling element 3 are formed of the stainless steel having thecomposition shown in table 3, and at least one of characteristic valuesincluding the average value of the circle equivalent diameters of theeutectic carbides contained in the stainless steel, the average area ofthe eutectic carbides, and the amount of retained austenite in thestainless steel (volume %), is outside of the range of the presentinvention.

In table 4, M and H of the anderon value represent a medium frequencyband (300 to 1800 Hz) and a high frequency band (1800 to 10000 Hz) ofmeasurement frequency bands, respectively. In an equal frequency band,lower anderon values represent better noiselessness.

The processability, the life, and the cost of the rolling bearing arerepresented by indices assuming that those of the prior art 1 are 100.Regarding the processability and the life, larger values indicate betterprocessability and longer life. Regarding the cost, smaller valuesindicate lower cost. The processability was evaluated in such a mannerthat peripheral cutting and parting cutting were conducted by precisionlathe and the resulting current increases were measured and compared.The life was evaluated in such a manner that, after heating the rollingbearing at temperatures such as 20° C., 80° C., and 100° C. for apredetermined time based on specification determined according to theuse, and rotating the rolling bearing for about 1000 hours in total, therotation condition (e.g., sound or vibration), grease condition, etcwere compared.

Table 4 shows the followings:

(1) Since the area ratio of the carbides, the maximum diameter of thecarbides, the average value of the circle diameter diameters of thecarbides, and the average area of the carbides of the contrast 2 are thesmallest, the corresponding anderon value is small. However, since thecontrast 2 using the conventional stainless steel for the outer andinner rings 1 and 2 requires special manufacturing processes to reducethese values, the manufacturing cost significantly increases. That is,the material of the contrast 2 is not economical.

(2) In the contrast 1 and the prior arts 1 and 2, the maximum diametersof the carbides are substantially equal to or shorter than those of theexamples 1 to 6, but the average value of the circle equivalentdiameters of the carbides and the average area of the carbides areoutside of the ranges of the present invention, and the anderon valuesare larger than those of the examples 1 to 6. In addition, in thecontract 1 and the prior arts 1 and 2, the costs are higher than thoseof the examples 1 to 6 to reduce the maximum diameters of the carbides.In particular, since the maximum diameters of the carbides of the priorart 2 and the contrast 1 are as small as 8 μm and 12 μm, respectively,and the average values of the circle equivalent diameters of thecarbides of the prior art 2 and the contrast 1 are relatively small, themanufacturing costs are extremely high.

(3) In the contrast 3, since the area ratio of the carbides, the averagevalue of the circle equivalent diameters of the carbides, the averagearea of the carbides, and the amount of retained austenite are outsideof the ranges of the present invention, the anderon values are extremelylarge.

(4) In comparison with the above contrasts and prior arts, the arearatio of the carbides, the average value of the circle equivalentdiameters of the carbides, and the average area of the carbides in theexamples 1 to 6 are all within proper ranges of the present invention.In addition, since other characteristics including the hardness of thecorrosion resistant bearing steel, the amount of retained austenite(volume %) in the corrosion resistant bearing steel, and the averagecrystal grain size of the corrosion resistant bearing steel in theexamples 1 to 6, are within proper ranges of the present invention, thecorresponding anderon values are substantially equal to that of thecontrast 2, and the processability, the life and the cost are all betterthan those of the prior arts.

FIG. 3 shows the relationship between the average values of the circleequivalent diameters and the anderon values (M). FIG. 4 shows therelationship between the maximum diameter and the cost index. In FIGS. 3and 4, “⊚” represent examples of the present invention, “▴” representcontrasts, and “●” represent prior arts. In FIGS. 3 and 4, the featureof the present invention is clearly shown, in which the anderon valuesand the costs are low.

As shown in FIG. 4, the cost of the contrast 3 is the lowest, but asshown in FIG. 3, the anderon value of the contrast 3 is extremely high.As shown in FIG. 3, the anderon value of contrast 2 is low, but as shownin FIG. 4, the cost of the contrast 2 is extremely high.

The area ratio of the carbides, the maximum diameter of the carbides,the average value of the circle equivalent diameters of the carbides,the average area of the carbides, and the average crystal grain size ofthe carbides, which are shown in table 2, were measured in such a mannerthat samples of the corrosion resistant bearing steels (or stainlesssteel) were buried in resin and ground, and the resulting samples wereobserved by metallic microscope and photographed at 400 magnification,and the images were measured by the image analysis device.

The eutectic carbide with the maximum diameter is least expected toappear on the ground surface. So, the rolling bearing was dissolved bygalvanostatic electrolysis in an acid solution, the carbides werefiltered by a filter, and the texture of the corrosion resistant bearingsteel was observed at 2000 magnification by scanning electron microscope(SEM). The results are shown in FIG. 5. In FIG. 5, white mass portionsrepresent the carbides.

The volume (%) of retained austenite in the corrosion resistant bearingsteel (or stainless steel) was measured by a surface X-ray diffractionspectroscopy after samples were treated by a electroextraction process.The analysis conditions were such that the target was Cu, theacceleration voltage was 40 kV, and the sample current was 180 mA. Thescanning range was 41.2 to 46. 705 degrees. The analysis method was suchthat a crystal structure was identified by integrated intensities ofdiffraction lines of Miller indices h, k, and 1, and a relative volumeratio of the amount of retained austenite in the corrosion resistantbearing steel was decided.

As a X-ray diffraction device, RINT1500/2000 type manufactured by RigakuDenki Corporation was used.

FIG. 6 is a perspective view showing an external appearance of a harddisk drive illustrating an example of an instrument including a rotatingportion to which the rolling bearing of the present invention can beapplied. In FIG. 6, reference numeral 11 denotes a disk, 12 denotes aspindle motor, 13 denotes a head, 14 denotes a suspension, 15 denotes aswing arm, 16 denotes a preamplifier, 17 denotes a flexer, 18 denotes aswing arm bearing, 19 denotes a voice coil motor, 20 denotes a frame,and 21 denotes an electric circuit.

The hard disk drive can be used in precision instruments such asvideotape recorder, or computer peripheral device. In addition, therolling bearing of the present invention can be used in a rotatingportion such as a spindle motor or fan motor. INDUSTRIAL APPLICABILITY

Since the present invention is constituted as described above, thepresent invention is especially suitable for use as a rolling bearingsuitable for use in a rotating portion of precision instrument such asvideotape recorder or computer peripheral device, a material for therolling bearing, and the instrument including the rotating portion usingthe rolling bearing.

1. A rolling bearing comprising a plurality of rolling elements providedbetween inner and outer rings, at least one of the inner and outer ringsbeing formed of corrosion resistant bearing steel comprising carbon of0.5 to 0.56 wt %, silicon of 1 wt % or less, manganese of 1 wt % orless, phosphorus of 0.03 wt % or less, sulfur of 0.01 wt % or less,chromium of 8.00 to 9.50 wt %, molybdenum of 0.15 to 0.50 wt %, copperof 0.30 to 0.7 wt %, titanium of 15 ppm or less, vanadium of 0.15 wt %or less, oxygen of 15 ppm or less, iron as remaining component andimpurities inevitably incorporated thereinto, the corrosion resistantbearing steel containing eutectic carbides having a circle equivalentdiameter with an average value of 0.2 to 1.6 μm, the eutectic carbideshaving an average area of 0.03 to 2 μm² and an area ratio of 2 to 7%,the corrosion resistant bearing steel having a hardness of HRC 58 to 62by JIS, and containing a retained austenite of 6 volume % or less.
 2. Arolling bearing comprising a plurality of rolling elements providedbetween inner and outer rings, the inner and outer rings, and therolling elements being formed of corrosion resistant bearing steelcomprising carbon of 0.5 to 0.56 wt %, silicon of 1 wt % or less,manganese of 1 wt % or less, phosphorus of 0.03 wt % or less, sulfur of0.01 wt % or less, chromium of 8.00 to 9.50 wt %, molybdenum of 0.15 to0.50 wt %, copper of 0.30 to 0.7 wt %, titanium of 15 ppm or less,vanadium of 0.15 wt % or less, oxygen of 15 ppm or less, iron asremaining component and impurities inevitably incorporated thereinto,the corrosion resistant bearing steel containing eutectic carbideshaving a circle equivalent diameter with an average value of 0.2 to 1.6μm, the eutectic carbides having an average area of 0.03 to 2 μm and anarea ratio of 2 to 7%, the corrosion resistant bearing steel having ahardness of HRC 58 to 62 by JIS, and containing a retained austenite of6 volume % or less.
 3. A rolling bearing comprising a plurality ofrolling elements provided between a rolling contact groove formed on anouter periphery of a shaft and a rolling contact groove formed on aninner periphery of an outer ring, at least one of the shaft and theouter ring being formed of corrosion resistant bearing steel comprisingcarbon of 0.5 to 0.56 wt %, silicon of 1 wt % or less, manganese of 1 wt% or less, phosphorus of 0.03 wt % or less, sulfur of 0.01 wt % or less,chromium of 8.00 to 9.50 wt %, molybdenum of 0.15 to 0.50 wt %, copperof 0.30 to 0.7 wt %, titanium of 15 ppm or less, vanadium of 0.15 wt %or less, oxygen of 15 ppm or less, iron as remaining component andimpurities inevitably incorporated thereinto, the corrosion resistantbearing steel containing eutectic carbides having a circle equivalentdiameter with an average value of 0.2 to 1.6 μm, the eutectic carbideshaving an average area of 0.03 to 2 μm² and an area ratio of 2 to 7%,the corrosion resistant bearing steel having a hardness of HRC 58 to 62by JIS, and containing a retained austenite of 6 volume % or less
 4. Therolling bearing according to claim 1, wherein an average crystal grainsize of the corrosion resistant bearing steel is 6 to 9.5 μm.
 5. Amaterial for a rolling bearing, which is corrosion resistant bearingsteel comprising carbon of 0.5 to 0.56 wt %, silicon of 1 wt % or less,manganese of 1 wt % or less, phosphorus of 0.03 wt % or less, sulfur of0.01 wt % or less, chromium of 8.00 to 9.50 wt %, molybdenum of 0.15 to0.50 wt %, copper of 0.30 to 0.7 wt %, titanium of 15 ppm or less,vanadium of 0.15 wt % or less, oxygen of 15 ppm or less, iron asremaining component and impurities inevitably incorporated thereinto,the corrosion resistant bearing steel containing eutectic carbideshaving a circle equivalent diameter with an average value of 0.2 to 1.6μm, the eutectic carbides having an average area of 0.03 to 2 μm² and anarea ratio of 2 to 7%.
 6. An instrument having a rotating portion usingthe rolling bearing according to claim
 1. 7. An instrument having arotating portion using the rolling bearing according to claim
 2. 8. Aninstrument having a rotating portion using the rolling bearing accordingto claim
 3. 9. An instrument having the rotating portion according toclaim 6 wherein the instrument is a hard disk drive.
 10. An instrumenthaving the rotating portion according to claim 6 wherein the instrumentis a precision instrument.
 11. The rolling bearing according to claim 2,wherein an average crystal grain size of the corrosion resistant bearingsteel is 6 to 9.5 μm.
 12. The rolling bearing according to claim 3,wherein an average crystal grain size of the corrosion resistant bearingsteel is 6 to 9.5 μm.
 13. An instrument having a rotating portion usingthe rolling bearing according to claim
 4. 14. An instrument having therotating portion according to claim 6 wherein the instrument is a harddisk drive.
 15. An instrument having the rotating portion according toclaim 7 wherein the instrument is a hard disk drive.
 16. An instrumenthaving the rotating portion according to claim 13 wherein the instrumentis a hard disk drive.
 17. An instrument having the rotating portionaccording to claim 8 wherein the instrument is a hard disk drive.
 18. Aninstrument having the rotating portion according to claim 7, wherein theinstrument is a precision instrument.
 19. An instrument having therotating portion according to claim 8, wherein the instrument is aprecision instrument.
 20. An instrument having the rotating portionaccording to claim 13, wherein the instrument is a precision instrument.